Combustion apparatus

ABSTRACT

A combustor having an array of dilution holes it provided with an asymmetrically located trim port for injection a controlled volume of air into the combustor. The controlled volume mixes with fuel rich air near the combustor wall and improves the dilution of the combustion gases. The improved dilution reduces the formation of NO x .

This invention relates to combustion apparatus and particularlycombustion apparatus in which dilution air is supplied to quenchcombustion products within a combustor.

Modern emission requirements set specific targets for the amount ofpollutants that may be ejected from combustion apparatus. Oneundesirable pollutant is NO_(x.) NO_(x) is produced particularly at hightemperature conditions where there is a slightly weaker thanstoichiometric mixture of air and fuel.

It is an object of the present invention to seek to provide an improvedcombustion apparatus giving reduced emissions.

According to a first aspect of the invention there is providedcombustion apparatus comprising a combustion envelope bounding acombustion volume the combustion volume having an upstream end and adownstream end, the combustion envelope having a plurality of rows ofdilution ports for the provision of dilution air which mixes in use in amidstream flow with hot combustion gas from a fuel injector, whereineach row extends in a circumferential direction and is spaced from anadjacent upstream or downstream row, the ports of each row being offsetcircumferentially from the dilution ports of an adjacent row,characterised in that at least one trim port is provided upstream of oneor more of the rows of dilution ports for supplying in use a flow oftrim cooling air into the combustion volume which mixes with hotcombustion gas between the midstream flow and an envelope cooling filmattached to a wall of the envelope.

Preferably the trim port is sized to limit the degree of penetration ofthe trim cooling air into the combustion volume.

The degree of penetration is preferably less than one third of half theradial height of the combustion volume.

The trim port may be between 0.35 and 0.7 of the diameter of one of thedilution ports.

Preferably the axis of the trim port is located downstream andcircumferentially offset from a dilution port in one or more of theplurality of rows.

Preferably the trim port is positioned to permit the flow of trimcooling air to be conveyed by swirl within the combustor to a locationdownstream and circumferentially aligned with a dilution port.

The combustion volume may be annular.

Preferably the combustion envelope has a fuel injection location,wherein a dilution port in the most upstream row is circumferentiallyaligned with the fuel injection location and the trim port is associatedwith the circumferentially aligned trip port.

The fuel injection location may be in an upstream wall of the combustionenvelope.

Preferably the combustion apparatus has two rows of dilution ports.

According to a second aspect of the invention there is provided a methodof improving combustion within a combustion envelope bounding acombustion volume the combustion volume having an upstream end and adownstream end and the combustion envelope having a plurality of rows ofdilution ports, characterised in that the method comprises providing atrim port wherein dilution air is supplied through the dilution portsand mixed with hot combustion gas in a midstream flow, wherein trimcooling air is supplied through the trim ports and mixed with hotcombustion gas upstream of one or the rows of dilution ports and outsidethe midstream flow.

Preferably swirl within the combustor conveys the trim cooling air to alocation downstream and circumferentially aligned with a dilution port.

The combustion apparatus may comprise a combustion envelope bounding acombustion volume the combustion volume having an upstream end and adownstream end, the combustion envelope having a plurality of rows ofdilution ports for the provision of dilution air into the combustionvolume, wherein each row extends in a circumferential direction and isspaced from an adjacent row in a direction perpendicular to thedirection in which each row extends, wherein the dilution ports of eachrow are offset circumferentially from the dilution port of an adjacentrow, characterised in that a trim port is provided between the first rowfrom the upstream end and the second row.

Embodiments of the invention will now be described by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 depicts a segment of an annular combustor in accordance with theinvention.

FIG. 2 depicts air flow through the dilution ports and trim ports.

FIG. 1 depicts a portion of an annular combustor 2. The combustor isused in conjunction with a fuel injector (not shown) that introducesfuel into the upstream end of the combustor. The fuel injector ispreferably of the type known as a “rich burn” injector which means thatit although it uses air to atomise fuel the majority of the air requiredfor combustion is provided through dilution ports 4, 6 on the radiallyinner and radially outer walls of the annular combustor.

The walls of the combustor themselves are kept cool by a film of airthat is effusion fed onto the wall by anangled hole that extends throughthe wall or a tile attached to the wall depending on the architecture ofthe combustor. As the cooling air is fed to the wall at an angle the airattaches to the wall to form a film that isolates the wall from hotcombustion gases. The film is thin and remains attached to the wall tillit is replenished by a new film from effusion cooling holes furtherdownstream. Although the volume of air used for wall cooling isrelatively small, once the film detaches from the wall it mixes with hotcombustion gases and can help lower the temperature of the main gas flowthrough the combustor.

A plurality of fuel injectors are mounted to the head end of thecombustor and are circumferentially spaced from one another at a uniformspacing.

The dilution ports 4, 6 are also arranged in circumferentially extendingrows with the ports through the radially inner wall and the radiallyouter walls being axially and circumferentially aligned. The ports ineach row are equally spaced and the ports of adjacent rows are offsetcircumferentially by half the port pitch. There are twice as manydilution ports (per row) as there are injectors, with the arrangementbeing that the first row of dilution ports spaced from the injector hashalf its dilution ports arranged to be circumferentially aligned withthe axis 8 of the injectors with the remaining half to becircumferentially intermediate the injector axes.

Accordingly, the configuration of ports results in a series of discreetcurrents of air 10, 12 that penetrate into the combustion space. Theflows coalesce 14 near the central region of the combustor when measuredbetween the radially inner and radially outer walls such that thepartially combusted, fuel rich mixture coming from the injectors isdiluted and further mixed with air flowing through the first (primary)row of ports and then further diluted and mixed with air flowing throughthe second (secondary) row of ports. The combustion products can bediluted in a controlled manner along the length of the combustor inorder to minimise both smoke emissions and NO_(x) production.

The streams of air entering the combustor through the dilution portshave a relatively high momentum ratio so that they can penetrate to thecombustor's mid stream. This maximises rapid dilution of the hot, fuelrich mixture in the central region of the combustor but the strongcurrents of cooling gad flowing through the dilution ports at a positionclose to the combustor walls interact poorly with the hot gas in thisregion. Poor interaction can result in little dilution in the axialplane which can persist downstream as streaks of hot gas thatsubsequently promotes NO_(x) formation in the combustor. The hot streaksalso contribute to an increase in the near wall radial temperatureprofile at the combustor exit. Since the combustion gases are feddirectly to the turbines significant air is required to control thetemperature to maintain a high turbine life.

To improve the mixing of the fuel rich gases between the midstream flowand the cooling film attached to the walls of the combustor a trim ports18 are provided. As minimum NO_(x) formation is typically required, boththe radially inner and radially outer combustor walls are provided withtrim ports to enable mixing both in radially inner and radially outerlocations.

The function of the trim port is to introduce a controlled amount of air16 into the combustion volume in the vicinity of the combustor wallsbetween the boundary cooling film and the combustor's midstream that isdiluted by the dilution air entering the combustor through the dilutionports 4, 6. This is typically a region from the wall cooling film to upto a third of half the combustor height. The relatively small volume ofair entering through the trim port does not have the momentum requiredfor it to penetrate to the centre of the combustor and is therefore fedto the near wall vicinity of the combustor to dilute and cool the hotgases at this location.

The degree of penetration of the air and the placement of its injectioninto the combustion volume is important as it controls the impact theair has on reducing local gas temperature. If the penetration is toostrong the jet will penetrate too deeply into the combustor volume andmerge with the main primary port flow as it mixes out in the mid-sectionof the combustor. If the penetration is too weak then the trim flow willbe absorbed into the near wall cooling film and not contribute todilution until much further downstream.

It is desirable that the trim port is located close to the primary portswhich are preferably at the same circumferential location as the fuelinjectors. The exact location of the trim port will depend on thespecific application and combustor geometry. The lower momentum of theair entering through the trim port enables swirl in the combustor tocarry the air that entered through the trim port circumferentially as itpasses axially down the combustor. Thus the swirl can carry the trimport air to pass downstream of the first row dilution port that liesdirectly in front of the fuel injector.

The optimum mixing point for the trim port air to mix with the hot gasoutside the core flow is just upstream of and between the secondaryports.

For an annular combustor the location for the trim port is typicallyfound on a line prescribed from a point between adjacent primary portsand the point at which the projected injector axis crosses thecircumferential line midway between the centre of the primary port rowand the centre of the secondary port row. Because the trim port airrequires a short axial distance to mix and spread the trim port ispreferably located either at the same axial spacing as the first row ofdilution ports or just downstream of these ports.

Although the trim ports can be located upstream of the primary dilutionports and still permit the swirl to carry the input air to the desiredmixing location there is a danger that the input air may become fouledwithin the dilution air which will adjust its path and can result in thetrim air being entrained in the core flow and failing to provide coolingto the hot gas in the near wall area. A greater than desirablecircumferential spacing between the trim port and primary dilution portmay be required to ensure such entrainment does not occur.

It will be appreciated that where the swirl direction is in a differentdirection the line 20 may be a mirror of its marked position whenreflected about the injector axis. A different swirl direction may beachieved by different fuel injector architectures.

Preferably the trim port diameter is between 0.35 and 0.7 of thediameter of the corresponding primary port.

In addition to a direct impact on NO_(x) control through reducing nearwall gas temperatures, the invention is also of assistance by improvingthe homogenisation of the near wall radial temperature profile at thecombustor exit. The more homogeneous temperatures allow the amount ofair used at this location to be reduced when compared with similarcombustors without the trim port. This reduction in the use of air atthis point allows it to be input into the combustor further upstreamwhere it will have a greater effect on NO_(x) formation.

Although the invention has been described with respect to annularcombustors it is equally applicable to can, or other forms of combustorshaving dilution ports.

1. Combustion apparatus comprising a combustion envelope bounding acombustion volume the combustion volume having an upstream end and adownstream end, the combustion envelope having a plurality of rows ofdilution ports for the provision of dilution air which mixes in use in amidstream flow with hot combustion gas from a fuel injector, whereineach row extends in a circumferential direction and is spaced from anadjacent upstream or downstream row, the ports of each row being offsetcircumferentially from the dilution ports of an adjacent row,characterised in that at least one trim port is provided upstream of oneor more of the rows of dilution ports for supplying in use a flow oftrim cooling air into the combustion volume which mixes with hotcombustion gas between the midstream flow and an envelope cooling filmattached to a wall of the envelope.
 2. Combustion apparatus according toclaim 1, wherein the trim port is sized to limit the degree ofpenetration of the trim cooling air into the combustion volume. 3.Combustion apparatus according to claim 2, wherein the degree ofpenetration is less than one third of half the radial height of thecombustion volume.
 4. Combustion apparatus according to claim 1, whereinthe trim port is between 0.35 and 0.7 of the diameter of one of thedilution ports.
 5. Combustion apparatus according to claim 1, whereinthe axis of the trim port is located downstream and circumferentiallyoffset from a dilution port in one or more of the plurality of rows. 6.Combustion apparatus according to claim 1, wherein the trim port ispositioned to permit the flow of trim cooling air to be conveyed byswirl within the combustor to a location downstream andcircumferentially aligned with a dilution port.
 7. Combustion apparatusaccording to claim 1, wherein the combustion volume is annular. 8.Combustion apparatus according to claim 1, wherein the combustionenvelope has a fuel injection location, wherein a dilution port in themost upstream row is circumferentially aligned with the fuel injectionlocation and the trim port is associated with the circumferentiallyaligned trip port.
 9. Combustion apparatus according to claim 8, whereinthe fuel injection location is in an upstream wall of the combustionenvelope.
 10. Combustion apparatus according to claim 8, wherein thecombustion apparatus has two rows of dilution ports.
 11. A method ofimproving combustion within a combustion envelope bounding a combustionvolume the combustion volume having an upstream end and a downstream endand the combustion envelope having a plurality of rows of dilutionports, characterised in that the method comprises providing a trim portwherein dilution air is supplied through the dilution ports and mixedwith hot combustion gas in a midstream flow, wherein trim cooling air issupplied through the trim ports and mixed with hot combustion gasupstream of one or the rows of dilution ports and outside the midstreamflow.
 12. A method according to claim 11, wherein swirl within thecombustor conveys the trim cooling air to a location downstream andcircumferentially aligned with a dilution port.